1. Field of the Invention
The present invention relates to a high frequency magnetic material and a high frequency circuit element including the same.
2. Description of the Related Art
Among circuit components for mobile communication devices such as mobile phones and wireless LAN, an inductance element and an impedance element are known. The inductance element is used as a component for impedance-matching circuits, resonant circuits and choke coils. The impedance element is used as a component for devices for suppressing noise, which is called electromagnetic interference and is hereinafter referred to as EMI. Since devices using high frequency have been increasing, it is also necessary for circuit components used for these devices to operate at a frequency of several hundred MHz to several GHz.
A hexagonal ferrite has been proposed as a material for devices that can operate at a frequency of several hundred MHz to several GHz. This material maintains permeability in a frequency band exceeding the frequency at which a spinel ferrite cannot maintain permeability. The hexagonal ferrite referred to herein is a magnetic material called a ferrox planar type ferrite, which has an easy magnetization axis in a plane perpendicular to the c-axis and was reported in the beginning of 1957 by Phillips Corporation.
A typical magnetic material of the ferrox planar type ferrite includes a Co-substituted Z type hexagonal ferrite expressed by the composition formula 3BaO.2CoO.12Fe2O3(Co2Z), a Co-substituted Y type hexagonal ferrite expressed by the composition formula 2BaO.2CoO.6Fe2O3(Co2Y), and a Co-substituted W type hexagonal ferrite expressed by the composition formula BaO.2CoO.8Fe2O3(Co2W).
Among the above ferrox planar type ferrites, the Y type hexagonal ferrite has a large anisotropic magnetic field perpendicular to the c-axis and has a large threshold frequency in the relationship between the frequency and the permeability. The Co-substituted W type hexagonal ferrite expressed by the composition formula BaO.2CoO.8Fe2O3(Co2W), which is typical of a Y type hexagonal ferrite, has a certain permeability at a frequency of up to several GHz and is therefore expected to be usable as a magnetic material for devices operating at a frequency of several hundred MHz to several GHz.
However, the firing temperature must be 1,150xc2x0 C., which is very high, in order that the ferrox planar type ferrite has a relative X-ray density of 90% or more. The relative X-ray density is herein defined as a ratio of the measured density of a sintered compact to the theoretical density, determined using X-rays.
Inductance elements and impedance elements are manufactured by firing green compacts including magnetic layers comprising a magnetic material and conductor layers comprising Ag or Agxe2x80x94Pd, which has a small relative resistance. Therefore, the diffusion of Ag and the destruction of the inner conductor must not arise in sintered compacts during the firing. It is thus necessary to use a magnetic material providing sintered compacts having a relative X-ray density of about 90% or more when the green compacts are fired at 1,100xc2x0 C. or less, and preferably at 1,000xc2x0 C. or less. When the sintered compacts have a relative X-ray density of about 90% or more, practical inductance elements or impedance elements can be manufactured in terms of the mechanical strength of elements.
A ferrox planar type hexagonal ferrite is disclosed in Japanese Unexamined Patent Application Publication No. 9-167703. However, it is not indicated in the publication that the hexagonal ferrite expressed by the composition formula (1-a-b)(Ba1-xSrx)O.aMeO.bFe2O3 or (1-a-b)(Ba1-xSrx)O.a(Me1-yCuy)O.bFe2O3, in which the ratio b/a is 2.2 or more to less than 3, can be sintered at low temperature. In the publication, substituting Ba with Pb is described but substituting Ba with Sr is not described. Effects obtained by firing the hexagonal ferrite in which Ba is substituted with Sr at low temperature are not also described.
Furthermore, a ferrox planar type hexagonal ferrite is also disclosed in Japanese Unexamined Patent Application Publication No. 9-246031. However, what is described in the publication is only how to sinter a Z type hexagonal ferrite at low temperature.
Accordingly, it is an object of the present invention to provide a high frequency magnetic material used for manufacturing an impedance element including an Ag or Agxe2x80x94Pd inner conductor and having the excellent characteristic of suppressing EMI at a frequency of several hundred MHz to several GHz.
It is another object of the present invention to provide a high frequency magnetic material including a Y or M type hexagonal ferrite which absorbs noise and has high sintered density and permeability in which the imaginary part xcexcxe2x80x3 is small at a frequency of less than 1 GHz and is large at a frequency of 1 GHz or more.
It is another object of the present invention to provide a high frequency magnetic material including a Y type hexagonal ferrite for impedance elements having high sintered density and the high Qm value (the ratio of the real part of the permeability to the imaginary part of the permeability) at a frequency of several GHz.
Furthermore, it is another object of the present invention to provide an inductance element and an impedance element operating at a frequency of several hundred MHz to several GHz using such a high frequency magnetic material.
In a first aspect of the present invention, a high frequency magnetic material includes a Y or M type hexagonal ferrite, wherein the hexagonal ferrite is expressed by the composition formula (1-a-b)(Ba1-xSrx)O.aMeO.bFe2O3, where Me is at least one selected from the group consisting of Co, Ni, Cu, Mg, Mn and Zn, 0.205xe2x89xa6axe2x89xa60.25, 0.55xe2x89xa6bxe2x89xa60.595, 0xe2x89xa6xxe2x89xa61 and 2.2xe2x89xa6b/a less than 3.
In a second aspect of the present invention, a high frequency magnetic material includes a Y or M type hexagonal ferrite, wherein the hexagonal ferrite is expressed by the composition formula (1-a-b)(Ba1-xSrx)O.a(Co1-yCuy)O.bFe2O3, where 0.205xe2x89xa6axe2x89xa60.25, 0.55xe2x89xa6bxe2x89xa60.595, 0xe2x89xa6xxe2x89xa61, 0.25xe2x89xa6yxe2x89xa60.75 and 2.2xe2x89xa6b/a less than 3.
In a third aspect of the present invention, a high frequency magnetic material includes a Y or M type hexagonal ferrite, wherein the hexagonal ferrite is expressed by the composition formula (1-a-b)(Ba1-xSrx)O.a(Co1-y-zCuyMez)O.bFe2O3, where Me is at least one selected from the group consisting of Ni, Mg and Zn, 0.205xe2x89xa6axe2x89xa60.25, 0.55xe2x89xa6bxe2x89xa60.595, 0xe2x89xa6xxe2x89xa61, 0.25xe2x89xa6yxe2x89xa60.75, 0 less than zxe2x89xa60.75, 0.25xe2x89xa6y+zxe2x89xa60.75 and 2.2xe2x89xa6b/a less than 3.
In a fourth aspect of the present invention, a high frequency magnetic material includes a Y or M type hexagonal ferrite, wherein the hexagonal ferrite is expressed by the composition formula (1-a-b)(Ba1-xSrx)O.a(Co1-y-zCuyZnz)O.bFe2O3, where 0.205xe2x89xa6axe2x89xa60.25, 0.55xe2x89xa6bxe2x89xa60.595, 0xe2x89xa6xxe2x89xa61,0.25xe2x89xa6yxe2x89xa60.75, 0 less than zxe2x89xa60.75,0.25xe2x89xa6y+zxe2x89xa60.75 and 2.2xe2x89xa6b/a less than 3.
The high frequency magnetic materials of the first to fourth aspects may further include about 0.1 to 30% by weight of Bi2O3.
In a fifth aspect of the present invention, a high frequency circuit element includes magnetic layers and internal electrode layers, wherein the high frequency circuit element is a sintered compact and the magnetic layers comprise the high frequency magnetic material according to any one of the first to fourth aspects.
A high frequency magnetic material of the present invention includes the hexagonal ferrite expressed by the composition formula (1-a-b)(Ba1-xSrx)O.aMeO.bFe2O3, in which the ratio b/a is 2.2 or more to less than 3. When a green compact includes the high frequency magnetic material, a sintered compact having a relative X-ray density of 90% or more can be obtained by firing the green compact at low temperature, for example, 1,100xc2x0 C. or less. The sintered compact includes a Y or M type hexagonal ferrite as a main phase. In the above formula, Me is at least one selected from the group consisting of Co, Ni, Cu, Mg, Mn and Zn. Among these metal elements, Co is the most preferable. When Me includes two elements, the combination of Co and Cu is preferable. When Me includes three elements, the combination of Co, Cu and Zn are preferable. The above elements are bivalent metals and have similar ion radiuses. Thus, when Me includes such elements, the effects of low temperature sintering can be obtained. For the bivalent metals, Co has an ion radius of 0.72 xc3x85, Ni has an ion radius of 0.69 xc3x85, Cu has an ion radius of 0.72 xc3x85, Mg has an ion radius of 0.66 xc3x85, Mn has an ion radius of 0.80 xc3x85, and Zn has an ion radius of 0.74 xc3x85. For other elements, Ba has an ion radius of 1.34 xc3x85, Sr has an ion radius of 1.13 xc3x85, Fe has an ion radius of 0.74 xc3x85, and O has an ion radius of 1.40 xc3x85.
For a high frequency magnetic material according to the present invention, it is confirmed that a sintered body contains a Y type hexagonal ferrite as a main phase on the basis of the X-ray diffraction analysis of the sintered body and the calculation of formula 1 below using the analysis data. Formula 1 shows the ratio of the X-ray diffraction peak intensity of a Y type hexagonal ferrite (205) plane to the total amount of peak intensity of the heterogeneous magnetoplumbite hexagonal ferrite (BaM, SrM) (114) plane, the BF phase (212) plane, the spinel ferrite (220) plane, the CuO (111) plane, and the hexagonal ferrite (205) plane. The Y type hexagonal ferrite includes (Co, Cu)2Y, the BF phase includes BaFe2O4, BaSrFe4O3 and the like, and the spinel ferrite includes CoFe2O4 and the like. In the present invention, a sintered body having a rate of 80% or more in formula 1 is defined as a Y type hexagonal ferrite. When the Sr-substituted rate is 100% (x=1), the main phase is a magnetoplumbite hexagonal ferrite and other phases are a spinel ferrite and BaSrFe4O3. The content of the magnetoplumbite hexagonal ferrite is determined using formula 1 in which the numerator is the peak intensity of the magnetoplumbite hexagonal ferrite (BaM, SrM) (114) plane. In the present invention, a sintered body having a rate of 60% or more in formula 1 having the above numerator is defined as an M type hexagonal ferrite.
xe2x80x83The crystallization ratio of Y type hexagonal ferrite=peak intensity of (Co,Cu)2Y(205) plane/peak intensity of {BaM(114) plane+BF(212) plane+spinel(220) plane+CuO(111) plane+(Co,Cu)2Y(205) plane}xe2x80x83xe2x80x83Formula 1
In the second aspect of the present invention, Me in the composition formula (1-a-b)(Ba1-xSrx)O.aMeO.bFe2O3 includes Co and Cu in appropriate contents. Therefore, the magnetic material can readily be sintered at low temperature and a sintered compact obtained by firing a green compact at 1,000xc2x0 C. or less has a relative X-ray density of 90% or more.
For a and b in the composition formula (1-a-b)(Ba1-xSrx)O.aMeO.bFe2O3, the ratio b/a is 2.2 or more to less than 3 and the formula is thus nonstoichiometric. When Me includes, for example, Co and Cu, low temperature sintering is allowed to proceed readily and the Y or M type hexagonal ferrite includes microcrystalline grains. Such a hexagonal ferrite has a large value of the product xcexcQ at a frequency of several hundred MHz to several GHz. The hexagonal ferrite is suitable for inductance elements and impedance elements for suppressing EMI at a frequency of several GHz or more.
In the formula of third and fourth aspects of the present invention, the following conditions are satisfied: 0.205xe2x89xa6axe2x89xa60.25, 0.55xe2x89xa6bxe2x89xa60.595, 0xe2x89xa6xxe2x89xa61, 0.25xe2x89xa6yxe2x89xa60.75, 0 less than zxe2x89xa60.75, 0.25xe2x89xa6y+zxe2x89xa60.75 and 2.2xe2x89xa6b/a  less than 3. Therefore, the formation of nonmagnetic spinel ferrites such as BaFe2O4 and SrBaFe4O8, which are crystal phases other than the Y or M type hexagonal ferrite, is suppressed. Thus, the real part xcexcxe2x80x2 of the permeability of the hexagonal ferrite is at least 2 at a frequency of 1 GHz. In a magnetic material of the present invention, there is a possibility that a small amount of crystalline BaFe2O4 and SrBaFe4O8 is formed but the threshold frequency in the relationship between the permeability and the frequency is enhanced up to several GHz.
In the fifth aspect of the present invention, the magnetic material further contains Bi2O3 at a certain content. When using such a magnetic material, ferrox planar type hexagonal ferrite devices having the following characteristics can be obtained: a Qm value of 40 or more at a frequency of several GHz and a relative X-ray density of 95% or more.
As described above, a high frequency magnetic material of the present invention can be used for devices operating at a frequency of several hundred MHz to several GHz. When a laminate includes magnetic layers and Ag or Agxe2x80x94Pd conductive layers each placed between the magnetic layers, such a laminate provides inductance elements and impedance elements operating at a frequency of several hundred MHz to several GHz.